U.S. Pat. No. 6,407,674, Gallagher discloses a reflectivity measuring apparatus for assessing the integrity of road markings including a light source, a light sensor and a processing means, the apparatus being mounted on a vehicle such that, in use, the light source illuminates a reflective surface on a road marking, and wherein the light sensor detects any reflected light and the data signal is received by the processing means, the processing means processes the data to provide information on the reflectivity of the reflective surface.
U.S. Pat. No. 6,674,878, Retterath discloses a system for the automated determination of retroreflectivity values for reflective surfaces disposed along a roadway. The system repeatedly illuminates an area along the roadway that includes at least one reflective surface using a strobing light source. Multiple light intensity values are measured over a field of view which includes at least a portion of the area illuminated by the light source. A computer processing system is used to identify a portion of the light intensity values associated with a reflective surface and analyse the portion of the light intensity values to determine at least one retroreflectivity value for that reflective surface. In particular, unilluminated images of reflective surfaces are acquired and used as reference images for determining the retroreflectivity value of the reflective surfaces in corresponding illuminated images.
WO2014/096398, Institute Of Technology Blanchardstown discloses multiline retroreflection measurement of road markings using a light source adapted to illuminate the road markings; a first camera adapted to measure the luminance of the road markings; a second camera adapted to make geometrical measurements of the road markings in combination with the first camera; and means for estimating the location of the measured road markings with respect to the first or second camera.
EP1486799 discloses an apparatus comprising an illuminator, a sensor comprising several adjacent detectors, means for controlling the illuminator, and a data processing and control unit. Retroreflection is determined from two photographs taken at substantially the same location. First a photograph is taken without illuminating the target with illuminator. After this, the illuminator is switched on and a second photograph is taken. Retroreflection can also be determined from one photograph, if the illuminator produces a thin strip of light. The luminance is then measured at the point illuminated by the strip of light and above and/or below it.
All of the references discussed above are incorporated herein by reference in their entireties.